As a sheet feeder for feeding a sheet of recording media in an image forming apparatus such as an electrophotographic copier, facsimile machine, or printer, a sheet feeder employing a friction method using a pickup member including rollers and a belt made of a material having a relatively high coefficient of friction, such as rubber, has been widely employed.
The configuration employing the friction method is relatively simple. However, the pickup member is pressed against the surface of the sheet by a spring or the like to obtain a relatively strong frictional force. Further, with material having a relatively high coefficient of friction, such as rubber, the coefficient of friction of the surface thereof changes with time or environment. With this method, therefore, it is difficult to obtain reliable sheet feeding performance.
Further, with printers in particular, diversification of users has brought about use of not only a plain sheet but also recording media sheets of various features, such as a coated sheet and a label sheet. Moreover the number and types of such recording media sheets are expected to continue to increase in the future. Some of these special-purpose recording media sheets have a surface with a substantially low coefficient of friction. Further, a release portion of the label sheet, for example, is removed in some cases by the rotating roller and the pressing member in the process of frictional separation. Therefore, there are cases in which it is difficult to separate sheets using conventional frictional separation.
The sheets difficult to separate by friction, as in the above-described example, may be separated by an air suction method that generates a negative pressure area by air suction and thereby attracts and conveys a sheet. This method provides relatively reliable sheet feeding performance compared to the friction method. The method, however, produces relatively large noise in air suction, and increases the size and cost of the device, and is therefore unsuitable for an appliance used in an environment such as an office.
To address the above-described issues, sheet feeders have been proposed which include an endless dielectric belt facing the upper surface of a stacked sheet stack and moving in the sheet feeding direction a charger to apply an alternating voltage to a surface of the endless dielectric belt to form thereon alternating charge patterns and to discharge the endless dielectric belt. The sheet feeders supply electrical charge to the surface of the endless dielectric belt, and generate an attraction force from an electric field generated by the electrical charge to thereby separate the uppermost sheet from the sheet stack and move the sheet in the sheet feeding direction.
Such a background sheet feeder includes, for example, a belt and a charging device. The belt made of a dielectric material is looped around rollers and faces the upper surface of a bundle of sheets loaded on a sheet loader. The charging device forms predetermined charge patterns on a surface of the belt. The sheet feeder attracts and feeds a sheet from the upper surface of the sheet stack using the belt, a fulcrum of which is set on the downstream side of the sheet in the sheet feeding direction. The belt swings about the fulcrum such that the surface of the belt facing the sheet is substantially parallel to the surface of the sheet loader facing the surface of the belt.
Another background sheet feeder includes a pickup member facing the upper surface of a stacked sheet stack and which moves in the sheet feeding direction, and picks up and feeds a sheet from the upper surface of the sheet stack using the pickup member. The pickup member includes an endless dielectric belt. The sheet feeder further includes a member that applies an alternating voltage to a surface of the endless dielectric belt. The member serves as a charging and discharging member for forming alternating charge patterns on the surface of the endless dielectric belt and discharging the endless dielectric belt.
Still another background sheet feeder attracts and feeds a sheet from stacked sheets using electrostatic force, and includes a rotatable endless dielectric belt, an electrostatic attraction device, and a contacting and separating device. The electrostatic attraction device includes a charging device that supplies charge to the outer circumferential surface of the endless belt. The contacting and separating device separately and swingably supports predetermined positions of the electrostatic attraction device in a direction substantially perpendicular to the sheet feeding direction using a pair of swing members.
In the above-described sheet feeders, however, sufficient sheet separation performance and sheet conveyance performance are not provided in some cases, depending on the properties of the sheet. It is therefore desired to provide a sheet feeder consistently discharging superior sheet separation performance and sheet conveyance performance.